Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed to measure oxygen to determine the proportion of combusted gases in an intake passage of the engine. The sensor may also be used during non-EGR conditions to determine the oxygen content of fresh intake air. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, when the intake oxygen sensor is used for EGR control, the EGR is measured as a function of the change in oxygen due to EGR as a diluent. To determine the change in the amount of oxygen, a reference point corresponding to an oxygen reading when no EGR is present is required. Such a reference point is called the “zero point” of the oxygen sensor. Due to the sensitivity of the oxygen sensor to pressure, as well as aging, there may be large deviations in the “zero point” at different engine operating conditions. In particular, aging and piece-to-piece variability may account for the biggest cause of change in the zero point oxygen reading. Therefore the oxygen sensor may need to be regularly calibrated and a correction factor may need to be learned.
One example method for calibrating an exhaust gas oxygen sensor is depicted by Ishiguro et al. in U.S. Pat. No. 8,417,413. Therein, a correction factor is learned based on an oxygen sensor output during engine fuel-cut off conditions. However, the inventors have recognized that approaches used for zero point estimation in exhaust oxygen sensors may not be applied for zero point estimation of intake oxygen sensors. This is because in addition to being sensitive to pressure and part-to-part variability, due to equilibration of the sensed gas by a catalyzing sensing element of the sensor, the oxygen sensor is also sensitive to the presence of fuel or other reductants and oxidants. As a result, the output of the intake oxygen sensor may be affected by the presence of purge hydrocarbons and/or positive crankcase ventilation gases received in the engine intake during the calibration conditions. The sensor measurements may be confounded by the various sensitivities, the accuracy of the sensor may be reduced, and thus, measurement and/or control of EGR, may be degraded.
In one example, some of the above issues may be addressed by a method for an engine comprising: learning a reference point for an intake oxygen sensor at a reference intake pressure during selected engine idling conditions; and adjusting EGR flow to the engine based on an intake oxygen concentration estimated by the sensor relative to the learned reference point, and further based on a change in intake pressure from the reference intake pressure. In this way, a zero point reading for an intake oxygen sensor may be learned more reliably, improving accuracy of EGR control.
For example, at the first engine idle following every engine start, an idle adaptation of the intake oxygen sensor may be performed. This may allow aging effects of the sensor to be learned. In addition, if a new sensor has been installed in the vehicle, the idle adaptation may be used to compensate for part-to-part variations. During the idle adaptation, an output of the intake manifold oxygen sensor may be monitored for a duration of the engine idling condition. A relationship between the output of the sensor at a reference intake pressure may be learned and corrected for factors such as humidity. When the idle adaptation is complete, the output of the intake oxygen sensor may be used to estimate an EGR concentration, and thereby adjust an EGR flow. Specifically, the output may be adjusted with a pressure correction factor based on the current intake pressure and the reference intake pressure, and the corrected oxygen sensor output may be used to more accurately estimate the change in intake oxygen concentration with EGR dilution. By correcting for pressure changes, the pressure effect on oxygen sensor readings is compensated for. In addition, by performing the adaptation during idling conditions, the effect of PCV and purge HCs on the oxygen sensor output is reduced.
In this way, a relationship between an intake oxygen sensor and an intake pressure sensor can be learned, independent of the accuracy of either sensor, and used to adjust EGR flow. By performing the learning during idling conditions, corruption of sensor output due to ingestion of PCV and purge HCs is reduced. By performing the learning during the first engine idle since an engine start, the effect of sensor aging on the sensor output can be learned. In addition, the relationship between the oxygen sensor output and the pressure sensor output can be learned under relatively consistent engine speed-load conditions. By also performing the idle adaptation each time a new oxygen sensor or pressure sensor is installed in the vehicle, the idle adaptation may be used to compensate for part-to-part variations. Overall, the accuracy of EGR estimation is increased, allowing for improved EGR control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.